Differences in the Effect of Dopamine on the Phototransduction Between Lampreys and Jawed Vertebrates.

Dopamine is one of the most important neurotransmitters for regulating retinal function and adjusting vision to the diurnal cycle. It exerts its regulatory effects, in part, through the cAMP pathway. Previous studies have demonstrated that dopamine affects phototransduction in amphibian rods, and that elevated intracellular levels of cAMP modulate the function of vertebrate rods and cones. Lamprey, the most primitive vertebrate, could be valuable for studying the evolution of dopamine regulatory loops in the retina. We examined whether the photoresponse properties of long (cone-like) and short (rod-like) photoreceptors in the river lamprey could be regulated by dopamine via the cAMP pathway. Using suction pipette recording, we demonstrated that forskolin-induced elevation of cAMP has no effect on long or short photoreceptors. At the same time, dopamine modifies the photoresponse properties of long, but not short, photoreceptors at high, potentially non-physiological concentrations. Immunohistochemical analysis of the lamprey retina revealed the expression of both D1 and D2 dopamine receptors in lamprey photoreceptors; however, their distribution differs from jawed vertebrates. Taken together, our results suggest that, in lampreys, dopamine does not regulate photoreceptor sensitivity to light in the circadian rhythm, but, rather, adjusts other retinal functions based on widespread distribution of its receptors.